tyres - probably the original Bridgestones - switch to the MPSS or similar - issue gone
exhaust - any aftermarket one is a 100% improvement
headliner - mines black - if you want a different colour - try a rag top - Will370z swears by 'em
350 comparison - never had one - can't comment - but loads of reviews on here
synchrorev - once you replace that zaust - you will never switch it off
speakers - there's one in the door and one in the side panel behind the seat - I find them fine (but I'm no connoisseur)